Port widget settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:

• Port to QQC2+Kirigami FormLayout style
• Use standard import names
• Adjust some strings to sound a bit more natural

Test Plan:
All features still work.

The Audio Volume tab was already ported earlier and is not included in this patch

Reviewers: VDG, Plasma, drosca

Reviewed By: drosca

Subscribers: abetts, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21105